Recently, I took on the challenge of making a pair of knitting needles for my wife - 5mm diameter, and 9" long. Obviously, as the diameter gets smaller, and the spindle gets longer, the tendency to 'whip' increases. I finished the project, but one of the needles snapped and went into the burn box.

There isn't a lot of information out there on turning knitting needles, but one of the YouTubes I found (un-narrated) appeared to show use of a file or rasp to finish turn the thin spindle. This technique make may sense - it imposes less stress on the piece, and spreads the 'cutting' over an area the width of the file so that it should be easier to get a uniform diameter, and files cut very gently, removing very small amounts of wood. Which means that it is necessarily slow - and that's not a bad thing.

Any adivce on using rasps or files as turning tools?

I suspect that the key to successfully turning knitting needles is use of a steady rest.

I have made a dozen or so and I leave 2" of thick wood on both sides and finish the small end first before going on. When I say finish the small end I do not mean part it off just leave it attached. Then I finish up towards the end of the head stock and when both are ready I part off the small end then take it to the bandsaw. Last I just use a carving chisel to make the hook and to slim the sides. then to 600grit paper. I learned how to do this by making a ton of hair sticks for the ladies at church.

Arlin's contribution reminded me of several videos I have seen of martini glasses with extremely thin stems. Lightly trap the tailstock end with a reverse cone or point and patiently shear cut very light cuts back towards the headstock.

IMO you can reduce (almost eliminate) the chance of splitting by using a riven blank.Blanks that are split along the grain, rather than a basic blank that's cut without regard to grain direction.Any hand tool that is subjected to some type of stress along the length, should be made with a riven handle. Striking tools, digging tools, prying tools, etc.While a knitting needle isn't a hammer handle, it's still prone to splitting during use.

Like Ed said, blanks should be split. Whipping comes from length, 10 to 1 ratio: blank 1 inch thick by 10 inches long will get almost no whip. Whip can be increased by too much pressure on the tailstock, so have just enough to secure the end. Also, whip comes from too much bevel rub: "the bevel should rub the wood but, the wood should not know it." This is the most difficult thing to learn about turning. Your hands on the tool should be "Hold the sword (tool) like you would a bird. Too tight and you kill it, too loose and it flies away." It takes a lot of practice to get there, and I am still working on it. Ashley Harwood is a master, turning her long finials. She does pretty much finish the bead on the end of the finial, and taper the blank to that bead, and then turns off the rest without using her finger on the back side as a steady rest. "Ashley, I have to go home and work on my dainty skills."